In conventional circuit breakers, the arc formed during a breaking operation is normally extinguished using compressed gas. The arc extinction or interruption performance is thereby mostly defined by the blow pressure and the physical properties of the medium, e.g. the dielectric strength, the heat capacity as a function of temperature, the electronegativity and the thermal conductivity. For large ratings, compressed sulphur hexafluoride (SF6) is generally used.
Typically, the arc interruption performance is improved by increasing the blow pressure of the gas using the self-blast or puffer principle. Although up to a certain rating the required interruption performance can be achieved, compressed-gas circuit breakers have intrinsic limitations that make it impossible to increase the performance without affecting product cost constraints.
Aiming at a reduction in the size, circuit breakers employing a liquefied gas, in particular SF6, as the interruption medium have been proposed, e.g. in U.S. Pat. No. 3,150,245. However, the design according to U.S. Pat. No. 3,150,245 has inter alia the drawback that given the low critical temperature of SF6 the respective storage vessel has to be designed for extremely high pressures.
In consideration of the drawbacks of this design, further circuit breaker using SF6 have been proposed in U.S. Pat. No. 4,288,668 and U.S. Pat. No. 4,307,274.
However, also the designs according to these documents have the drawback that a sophisticated, external pressurization and cooling system is required to keep SF6 in its liquid phase. As a result, the cost both for the construction and the operation of such liquefied-gas circuit breakers are substantial and they have thus not found acceptance yet.